(if I understand your situation correctly)
https://www.rdocumentation.org/packages/seqinr/versions/3.6-1/topics/read.alignment shows how to use the function read.alignment which can take fasta msf etc. The docs provide the example'
read.alignment(file = system.file("sequences/LTPs128_SSU_aligned_First_Two.fasta", package = "seqinr"), format = "fasta", whole.header = TRUE) but you can use this code below (assumes those files are aligned) to go from reading the tree to getting the distances, producing the neighbor joining phylogenetic tree, and then plotting the tree.
fasta.res <- read.alignment(file = "geneticAlignment.msf", format = "fasta")
fasta.res.dist.alignment = dist.alignment(msf.res, matrix = "identity")
fasta.res.dist.alignment.nj = nj(fasta.res.dist.alignment)
plot(fasta.res.dist.alignment.nj, main = "from fasta files")